The invention relates to a process for regenerating the loaded scrubbing medium in a physical gas scrub in which at least materials of a first type and a second type are scrubbed out of a gas mixture, where predominantly materials of the first type are separated off from scrubbing medium loaded with materials of the first type and second type by stripping at elevated pressure and a loaded scrubbing medium enriched in materials of the second type and also a gas phase comprising the stripping gas used in stripping and the materials separated off from the loaded scrubbing medium are obtained.
The invention further relates to an apparatus for carrying out the process of the invention.
Physical gas scrubs are used for the separating gas components from gas mixtures. They utilize the ability of a liquid used as scrub medium to absorb gaseous materials and keep them in solution without these materials being chemically bound. How well a gas is absorbed by a liquid and bound by the latter can be expressed by its solubility coefficient, the better the gas dissolves in the liquid, the greater is its solubility coefficient. The solubility coefficient is temperature dependent and generally increases with decreasing temperature.
To remove the gas components which have been separated off from the gas mixture and dissolved in the scrubbing medium, the loaded scrubbing medium is regenerated after the gas is scrubbed. The regenerated scrubbing medium is normally reused for the gas scrub, while the gas components which have been separated off are either disposed of or passed to a profitable use.
In order to purify crude synthesis gasses, gas mixtures which are produced on an industrial scale in gasification plants from coal or/and hydrocarbon-containing feeds, for example by reforming by means of steam or by partial oxidation and generally contain not only the desired materials hydrogen (H2) and carbon monoxide (CO) but also some undesirable constituents such as carbon dioxide (CO2) and the sulfur components hydrogen sulfide (H2S) and carbon oxide sulfide (COS), preference is given to using physical gas scrubs. These processes are attractive since the crude synthesis gasses are nowadays usually produced under high pressure and the effectiveness of physical gas scrubs increase, to a first approximation, linearly with the operating pressure. The methanol scrub is of particular importance for the purification of crude synthesis gasses. It makes use of the fact that the solubility coefficients of H2S, COS and CO2 in methanol differ greatly from those of H2 and CO. Since these differences increase and the H2 and CO losses by co-absorption thus decrease with decreasing temperature, but especially because the solubility coefficient of CO2 increases greatly with decreasing temperature, the methanol scrubbing medium is usually introduced at a temperature far below 0° C. into a scrubbing column and brought into intimate contact with the crude synthesis gas to be purified. In addition, this process allows independent isolation of sulfur components and of carbon dioxide since the sulfur components have considerably greater solubility coefficients than carbon dioxide and can therefore be separated off largely selectively from the loaded methanol scrubbing medium.
If production of a hydrogen product is a primary objective, the crude synthesis gas is usually subjected to a water gas shift, in which the carbon monoxide present in the crude synthesis gas is reacted with water to form hydrogen and carbon dioxide, before the methanol scrub. Particularly when the crude synthesis gas is obtained from a low-hydrogen feed such as coal or heavy oil and therefore has a high CO2 content even before the water gas shift, the CO2 content can increase to more than 40 mol % after the water gas shift. In order to improve the economics of hydrogen production, a by-product which, for example, with a purity of more than 98 mol % can be used for the synthesis of urea is frequently isolated from the carbon dioxide which is formed in large quantities.
To obtain the CO2 by-product, methanol scrubbing medium loaded with carbon dioxide and also the more soluble sulfur components which are therefore more strongly bound to the scrubbing medium is, according to the prior art, depressurized to a moderate pressure in the lower part of a CO2 product column configured as scrubbing column. Owing to the pressure reduction, predominantly CO2 but also part of the sulfur components outgas from the loaded methanol scrubbing medium and are conveyed upward in the scrubbing column. The sulfur components are scrubbed out of the gas stream by means of a sulfur-free methanol scrubbing medium which for this purpose is introduced at the top of the CO2 product column and is brought into intimate contact with the ascending gas stream, A CO2 stream having product purity is take off from the top of the CO2 product column while a methanol scrubbing medium which has a greatly reduced CO2 content but is still loaded with CO2 and the sulfur components accumulates in the lower region of the column.
The methanol scrubbing medium which is still loaded with carbon dioxide and sulfur components is taken off from the CO2 product column and introduced into an enrichment column which is a cold stripping column. In the enrichment column, predominantly CO2 is stripped from the methanol scrubbing medium by means of a stripping gas, usually nitrogen, conveyed in counter current, so that a carbon dioxide-rich gas phase which despite being contaminated with stripping gas often has, after backscrubbing of the sulfur components present, a CO2 purity which allows it to be passed on as CO2 product can be taken off from the top of the enrichment column.
The methanol scrubbing medium which after stripping is loaded predominantly with sulfur components but also with a residual amount of CO2 is taken off from the enrichment column and conveyed to hot regeneration [in which a methanol fraction of scrubbing medium purity and a gas fraction containing sulfur components (sour gas fraction),] the CO2 content of which is determined mainly by the CO2 content of the loaded methanol scrubbing medium achieved in the enrichment column, are produced. Since the sulfur present therein is a raw material which is prized in the industry, the sour gas fraction has economic potential. In order to realize this potential, the sulfur components present in the sour gas fraction are usually converted in a Claus process into elemental sulfur, but if this is to be possible the CO2 content of the sour gas fraction must not exceed a limit value.
Particularly when the crude synthesis gas to be purified has a low sulfur content, as is the case, for example, for crude synthesis gas produced by gasification of low-sulfur coal, this limit value can only be achieved, if at all, with a great outlay, e.g. by cold stripping with an increased amount of stripping gas. However, if the carbon dioxide-rich gas phase obtained contaminated with stripping gas during stripping is to be passed on as CO2 product, only a limited increase in the amount of stripping gas is possible because of the required product purity.
In the patent application DE102006049602, the disclosure content of which is incorporated in full into the description by reference, the loaded scrubbing medium is, after cold stripping, taken off from the enrichment column, warmed and subsequently subjected to warm stripping, in which the CO2 content of the loaded scrubbing medium is reduced further and a CO2-containing gas fraction is produced. The CO2-containing gas fraction is subsequently recirculated to the enrichment column and goes via this into the tailgas to be discarded.
However, to achieve economical synthesis gas production, it is critical for the carbon dioxide to be separated off very completely from the loaded methanol scrub medium not only with a small outlay but also to be converted with a high percentage and appropriate purity into a CO2 product.
It is therefore an object of the present invention to provide a process of the type mentioned at the outset and also an apparatus for carrying out the process which allow the disadvantages of the prior art to be overcome.